Plasma processing has long been employed to process substrates (e.g., wafer or flat panels or other substrates) to create electronic devices (e.g., integrated circuits or flat panel displays). In plasma processing, a substrate is disposed in a plasma processing chamber, which employs one or more electrodes to excite a source gas (which may be an etchant source gas or a deposition source gas) to form a plasma for processing the substrate. The electrode may be excited by an RF signal, which is furnished by an RF generator, for example.
In some plasma processing systems, multiple RF signals, some of which may have the same or different RF frequencies, may be provided to the substrate-bearing electrode (also referred to herein as the lower electrode or chuck) to generate plasma while the upper electrode is grounded. In a capacitively-coupled plasma processing system, for example, one or more RF signals may be provided to the bottom electrode while the top electrode is grounded.
In some applications, the plurality of RF signals may be pulsed. For any given RF signal, RF pulsing involves turning the RF signal on and off (or alternating between a high power level and a low power level since pulsing does not always require the power to be turned off) at a pulsing frequency that may be different from (and typically slower than) the RF frequency. Generally speaking, RF pulsing is performed in the past to improve certain processing results (such as to improve uniformity or reduce etching-related damage).
The pulsing of the various RF signals may be unsynchronized or synchronized. With respect to synchronized pulsing, for example, if two signals RF1 and RF2 are synchronized, there is an active pulse of signal RF1 for every active pulse of signal RF2. The pulses of the two RF signals may be in phase, or the leading edge of one RF pulse may lag behind the leading edge of the other RF pulse, or the trailing edge of one RF pulse may lag behind the trailing edge of the other RF pulse, or the RF pulses may be out of phase.
If the pulsing of the various RF signals is not well-controlled, there is a risk that RF power instability resulting in plasma perturbation may occur during the transition from low to high (or vice versa) of one or more of the RF signals. This is because during such a transition by one or more of the RF signals, the plasma condition in the processing chamber changes. Such change may be detected by the match network and/or the other RF generators, which may attempt to compensate for the detected plasma condition changes. The reactive nature of such compensation means that for the duration between a plasma condition change detection and successful compensation, RF power perturbations resulting in plasma instability exist.
FIG. 1 shows an example of such RF power perturbation, which may result in plasma instability during the transition of one of the pulsing RF signals. In the example of FIG. 1, the 2 MHz RF signal pulses at 100 Hz with a 50% duty cycle between 2,500 W and 0 W. For illustration purposes, suppose the 60 MHz RF signal operates in the continuous waveform (CW) mode without pulsing. As the 2 MHz RF signal transitions from the low state 102 to the high state 104, the plasma condition within the chamber changes in response to the changing power supplied. The 60 MHz RF signal, upon detecting such plasma condition change, is shown compensating (either via the compensation circuit in the 60 MHz RF power supply or in the match network) for the detected plasma condition change.
However, this is a reactive response and depends on first detecting the plasma condition change brought about by the low-to-high transition of the 2 MHz pulsing RF signal (which pulses at a pulsing frequency of 100 Hz as mentioned earlier). The delay and subsequent response causes the RF power level perturbation shown by reference number 106, which shows a temporary dip in the power level of the 60 MHz RF signal after the 2 MHz transitions from low to high. Another instance of RF power level perturbation in the 60 MHz RF signal due to the delayed response of the 60 MHz RF signal is shown by reference number 108 after the 2 MHz RF transitions from high (110) to low (112). Other RF power perturbations are shown by reference numbers 114 and 116 in FIG. 1, for example. As can be seen in FIG. 1, these RF power perturbations may be in the positive direction or negative direction and may have different intensities. Such perturbations result in unstable and/or poorly controlled plasma events, affecting process results and/or device yield.
Furthermore, modern plasma processes impose stringent process result requirements in the fabrication of high density, high performance devices. Some process windows cannot be reached or are quite narrow with traditional constant waveform RF signals or with traditional RF pulsing methods.
Manipulating and further control of the pulsing of various RF signals to improve plasma stability and/or to provide additional process control knobs are among the many goals of embodiments of the present invention.